I emailed Pilot, and the pens are acid free/archival quality- so you don't need to worry about them leaving something nasty in your quilt!
(They did include a "but don't use these for archiving- they disappear" note. Well, that's exactly what I want!)
I don't do show quilts, so these are actually PERFECT for art quilts- things that don't get washed. I've never figured out how to wash off the marks I do use (even just chalk) if I don't run it through the wash, and the air soluble markers don't last long enough.
I'll likely use them on my regular quilts too- since those all go into the washing machine.
Knowing that they are acid free makes me very happy, and seems to have been a concern for many (at least in older threads)